The research was conducted by Northwestern University in Illinois. (representative)
Illinois:
New research from Northwestern University has linked subsurface climate change to the shifting earth beneath urban areas. As the soil warms up, it also deforms. These phenomena result in excessive ground movement and cracking due to expansion and contraction, ultimately impairing the long-term durability and operational performance of structures.
Researchers argue that such rising temperatures may have contributed to earlier building deterioration, and they expect these problems to continue for years to come.
While rising temperatures pose a threat to our infrastructure, the researchers also see it as a potential opportunity. By capturing the waste heat emitted underground from underground transportation systems, parking garages and basement facilities, urban planners can mitigate the effects of subsurface climate change and reuse the heat in an untapped thermal energy resource.
The study is published today in Communications Engineering, a Nature Portfolio journal. It is the first study to quantify ground deformations caused by underground heat islands and their effect on civil infrastructure.
“Underground climate change is a silent danger,” said Northwestern’s Alessandro Rotta Loria, who led the study. “The ground deforms due to temperature variations and no existing civil structure or infrastructure is designed to withstand these variations. While this phenomenon is not necessarily dangerous to people’s safety, it will affect the normal day-to-day operation of foundation systems and civil infrastructure in general,” said Mr. Rotta Loria.
“Chicago clay can contract when heated, just like many other fine-grained soils. Due to increases in subsurface temperatures, many downtown foundations are slowly but continuously undergoing unwanted settlement. In other words, you don’t have to live in Venice to live in a city that is sinking — even though the causes of such phenomena are completely different,”
Mr. Rotta Loria is an assistant professor of civil and environmental engineering at Northwestern’s McCormick School of Engineering.
What is Subsurface Climate Change? In many urban areas around the world, heat is continuously spreading from buildings and underground transportation, warming the ground at an alarming rate. Previous researchers have discovered that the shallow subsurface beneath cities is warming by 0.1 to 2.5 degrees Celsius per decade.
Known as “subterranean climate change” or “subterranean heat islands,” this phenomenon is known to cause ecological problems (such as contaminated groundwater) and health problems (including asthma and heatstroke).
But until now, the effect of subsurface climate change on civil infrastructure has been unstudied and little understood.
“If you think of basements, parking garages, tunnels and trains, all these facilities are continuously emitting heat,” says Rotta Loria.
“In general, cities are warmer than rural areas because building materials periodically trap heat derived from human activity and solar radiation and then release it into the atmosphere. That process has been studied for decades. Now we’re looking at its underground counterpart, which is largely driven by anthropogenic activity,” he added.
Slowly sinking
After three years of collecting temperature data, Mr. Rotta Loria built a 3D computer model to simulate how ground temperatures have evolved from 1951 (the year Chicago completed its subway tunnels) to today. He found values consistent with those measured in the field and used the simulation to predict how temperatures will evolve up to the year 2051.
Mr Rotta Loria also modeled how soil deforms in response to rising temperatures. While some materials (soft and stiff clay) contract when heated, other materials (hard clay, sand and limestone) expand.
According to the simulations, higher temperatures could cause the soil to swell and expand up to 12 millimeters. They can also cause the ground to contract and sink as much as 8 millimeters – under the weight of a building.
While this may seem subtle and unnoticeable to humans, the variation is more than many building components and foundation systems can handle without compromising their operational requirements.
“Based on our computer simulations, we have shown that ground deformations can be so severe that they lead to problems for civil infrastructure performance,” said Rotta Loria.
“It’s not like a building suddenly collapses. It sinks very slowly. The consequences for the usability of structures and infrastructures can be very serious, but it takes a long time to see them. It is very likely that underground climate change has already caused cracks and excessive foundation settlement that we didn’t associate with this phenomenon because we weren’t aware of it,” he added.
Harvesting heat
Because urban planners and architects designed most modern buildings before subsurface climate change emerged, they failed to design structures that could withstand the temperature swings we experience today. Still, modern buildings will outperform buildings from earlier periods, such as the Middle Ages.
“In the United States, the buildings are all relatively new,” said Rotta Loria.
“European cities with very old buildings will be more sensitive to subsurface climate change. Buildings made of stone and bricks that resort to past design and construction practices tend to be in a very delicate balance with the disturbances associated with the current activities of cities. The thermal disturbances associated with subsurface heat islands could have detrimental effects on such structures,” he added.
Going forward, Mr. Rotta Loria said future planning strategies should integrate geothermal technologies to harvest waste heat and deliver it to buildings for space heating. Planners can also install thermal insulation on new and existing buildings to minimize the amount of heat entering the ground.
“The most effective and rational approach is to insulate underground structures in a way that minimizes the amount of wasted heat,” said Rotta Loria. “If that is not possible, geothermal technologies offer the possibility to absorb heat efficiently and reuse it in buildings. What we do not want is for technologies to be used to actively cool underground structures, because that costs energy. Currently, there are countless solutions that can be implemented,” he added.
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